Antagonistic means actively opposed or hostile to someone or something. When two people are antagonistic toward each other, they're working against each other, often with real hostility, treating each other as enemies rather than simply having different views. Think of two students who used to be friends but now can't stand each other, making sarcastic comments and trying to undermine whatever the other person does.

The word comes from the idea of an antagonist, which is the opponent or enemy in a story. Just as the antagonist in a novel opposes the hero at every turn, antagonistic people or forces work in opposition. Countries might have an antagonistic relationship if they constantly oppose each other's policies. Scientists might describe two chemicals as antagonistic if they cancel each other out or work against each other's effects.

Being antagonistic is more intense than simply having a different opinion. You can disagree with your friend about which movie to watch without being antagonistic. But if you start mocking their choice, trying to convince everyone else it's terrible, and refusing to compromise, then you've crossed into antagonistic behavior. The word carries a sense of active opposition rather than passive disagreement.